I wasn't referring to the culture of the platform, so much as the culture of the userbase.
Besides, Facebook is anything but a monoculture - nothing with a billion users could be considered a monoculture, especially not a platform where people maintain their real identity and culture rather than adopting a "platform" culture like on Reddit or 4chan.
On the contrary the Facebook of 2020 is far more a monoculture than the internet of 2000 or 1990.